The main circuit connection modes and operating voltage characteristics of VSC-HVDC system have brought new technical problems on equipment type selection. Aiming at symmetric monopole connection mode and based on the asynchronous interconnection project between Yunnan power grid and CSG, the key type selection of connected transformer for VSC-HVDC system is systemetically studied, a technical principle is proposed to determine the valve side insulation tolerance level of connected transformer by calculating the maximum load voltage drop based on reactive power controlled mode, the recommended process and methods of insulation parameters configuration are also given, and the relative insulation test method for winding terminal is put forward. Furthermore, considering that the symmetrical bipolar connection mode may be used in VSC-HVDC system with higher voltage level and lager current in the future, the paper proposes a calculation formula to calculate the DC and AC withstand voltage at valve side winding. The research results have important technical guiding significance for the type selection, technical standards formulation and test of the connected transformer.